Geochemical Constrains On The Opening And Closing Of The South China Sea

The opening of the South China Sea?SCS?was one of the most significant tectonics surrounding the SE Asia during the Cenezoic.Modes of the opening of the SCS,that the continental rifting was propagated westward and the³3-15 Ma seafloor spreading centre was moving southward,were established by the proceedings of the Major Research Program�The South China Sea Deep�.Moreover,the tomography reveals that the SCS previously extended⁴00 to 500 km east of the present Manila trench.Nevertheless,the less constrained is which dynamic modes,including?1?the backarc spreading of the Andean-type proto-South China continental margin,?2?the lateral extrusion of the Indochina block,?3?the slab pull caused by the southward subduction of the proto-SCS,and?4?the upwelling of the Hainan plume,predominated the opening of the SCS.Furthermore,when and how did the SCS subduct beneath the Philippine Sea Plate is less known.In this thesis,mafic samples including?1?the in situ middle Miocene MORB crust and the late Miocene seamount basalt from the SCS east subbasin,?2?the East Taiwan Ophiolite,Taiwan Island,?3?the Amnay Ophiolite,Mindoro Island,Philippines,and?4?ophiolitic gravels of the Kenting M�lange,South Taiwan,have been investigated through dating and geochemical survey.The dynamic modes of the opening and closing of the SCS have been discussed in conjunction with the new results of this thesis and previous results of other Cenozoic igneous rock located inside and around the SCS.The diverse geochemical and isotopic signatures of the middle Miocene MORBs of the SCS cored by the IODP Expidition 349 reveal that the mantle source supported the middle Miocene seafloor spreading of the SCS was compositionally heterogeneous.Exactly,the SCS southwest subbasin is composed of elementally enriched MORB?E-MORB?,whereas the SCS east subbasin consists of elementally depleted nomal MORB?N-MORB?dominatly and little of E-MORB.However,the E-MORBs of the southwest subbasin are isotopically depleted than the N-MORBs of the east subbasin.The late Miocene seamount basalt dredged from the Scarborough seamount chain is elementally and isotopically less enriched than the dominant OIB-type seamount basalts of the SCS and the Hainan continental flood basalts.Perhaps this less enriched OIB-type seamount basalt was resulted from the dilution of the Hainan plume material by the depleted MORB mantle?DMM?of the SCS.We proposed that the off-axis elementally enriched Hainan plume materials underwent multiple extractions of incompatible elements due to decompression melting during their long journey to the Miocene SCS mid-ocen ridge.Therefore,the middle Miocene compositionally heterogeneous MORBs of the SCS can be referred to various degrees of influx of elementally less enriched Hainan plume materials into the DMM-like mantle domain.Zircon U-Pb dating indicting that the East Taiwan Ophiolite?ETO?was crystallized at¹7 Ma.The mafic rocks of the ETO differ significantly in incompatible elements and Sr-Nd isotopes from the middle Miocene MORBs of the SCS east subbasin.In contrast,the selected enrichement of Rb-Ba-Pb-Sr and negative Nb-Ti anomalies of the mafic rocks of the ETO are consistent with the forearc basalts?FAB?of the Izu-Bonin-Mariana?IBM?forearc.The low in situ zircon O isotopic values(?¹⁸O=+3.47�to+4.97�)of the ETO gabbo indicate a wet mantle source with high contents of hydrous fluid,supporting a SSZ tectonic regime.Whereas,the ETO adakitic plagiogranite and its DMM-like in situ zircon O isotopic values(?¹⁸O=+3.65�to+5.49�)reveal a petrogenesis of the partial melting of the SCS subducting slab during its initial subduction at¹7 Ma.Differing from the slab sinking of the old Pacific along Mariana Trench,the subduction of the SCS is an induced nucleation responded to continuing plate convergence from the northwestward drifting of the West Philippine Sea Plate.The late Oligiocene ophiolitic gravels of the Kenting M�lange,South Taiwan,display backarc basin basalt?BABB?-like incompatible elements and DMM-like Nd-Hf isotopes.Their distict geochemical signatures from the FABs of the North Luzon Trough?i.e.the ETO?and the MORBs of the Huatung basin support that most of them are fragments of off-scraped SCS oceanic crust.Meanwhile,their BABB-like incompatible elements argue against there was source mixing between the Hainan plume and the SCS mid-ocean ridge during the Oligocene period.Integrating with other tectonics,the head of Hainan plume might impinge the lithosphere of the South China continental margin at the middle Oligocene,and the arriving of Hainan Plume has contributed to the latest Oligocene/earliest Miocene ridge jump and propagation.Since the SCS developed upon the late Mesozoic Andean-type South China continental margin,the BABB-like signatures of the ophiolitic gravels might be related to an earlier metasomatism of the DMM-like mantle caused by the Mesozoic subduction of the paleo-Pacific Plate.Zircon U-Pb dating indicting that the Amnay Ophiolite of the Mindoro Island was crystallized at³3-23 Ma.The Amnay Ophiolite consists of both N-MORB-type and E-MORB-type basalts based on trace element compositions.The N-MORB-type samples display obvious negative Nb anomaly and the highly enriched Rb-Ba-Th,which are consistent with a SSZ-type tectonic regime.Furthermore,the distinct in situ zircon O isotopes from the mantle zircons reveal contributions from high contents of hydrous fluid and the recycled sediments.The E-MORBs of the Amnay Ophiolite share similar Nd-Hf isotopes with N-MORB-type samples.Both the N-MORB-type and E-MORB-type samples are clearly different from the late Oligocene BABB-like MORBs of the SCS?i.e.the ophiolitic gravels of the Kenting M�lange?.A forearc tectonic setting which corresponded to the Oligocene southward subduction of the proto-SCS is proposed due to?1?the N-MORB-type samples share similar geochemical signatures with FABs of the Izu-Bonin-Mariana forearc and?2?the Oligocene Amnay Ophiolite seems to be cogenetic with the Cagayan Ridge and the SE Sulu Sea.The timing of the onset of strike-slip movement along the Red River shear zone coincides well with the timings of the initiation of subduction in Palawan and the initial seafloor spreading of the SCS.Perhaps the hard continental collision between the Indo and the Eurasia was the driving force to extrude the Indochina block,open the SCS,and subduct the proto-SCS.In consideration of the middle Oligocene upwelling of the Hainan plume,the Paleocene and Eocene rifting of the Andean-type proto-South China continental margin require some much earlier dynamic forces.It has been proposed that the late Mesozoic retreat of the western Pacific subduction zones made backarc extension along the Andean-type proto-South China continental margin.Furthermore,studies of seismic anisotropy and numerical simulations have suggested that the Paleocene and Eocene subduction of the Indo continent may have driven significant lateral mantle flow under the Asian continent and raised the diffuse asthenospheric mantle.Obviously,the subduction slab penetrated beneath the SE Asia could hinder the lateral escape of the asthenospheric mantle,leading to robust asthenospheric upwelling,continental rifting,and widespread Cenozoic volcanism.Consequently,the Paleocene and Eocene OIB-type volcanism in South China continental margin could have derived from the upwelling of enriched asthenosphere.At the mean time,the upwelling asthenosphere heated and replaced the ancient sub-continental lithospheric mantle?SCLM?of South China,and partial melting of the upper highly metasomatic SCLM layer produced the Paleocene and Eocene IAB-type volcanism.Whereas,the lower less metasomatic depleted SCLM layer might have formed the DMM-like asthenospheric mantle source of the SCS.